First you argue that the device is secure. When I point out that it is not, you switch tack to argue that when you take other factors into account, it's still "better" than doing it some other way because of some other factor, so overall security is good.

You also want to conveniently forget aspects of your original argument at your blog, such as the defect with OTP that a phone can be stolen. A phone is much, much, much more secure than u2fzero.

You're also insisting that your particular use case for needing a backup is the only and best use case. If you want to design a system for your individual desire and needs, that's great, but this doesn't generalize. I'm guessing most people will not want their device baseline to be insecure so that they can have a 2nd insecure backup at hand.

As I hinted, the best solution is likely a cloud based token. This can be simulated with u2fzero by using a method similar to what @ecesena proposed. With either knowledge of an authorized external transport key, or by sharing a transport key, the atecc508a can share key material securely. I won't work out the details here, but basically you just need a single initialization-time secret that you store in the cloud. With that, you can take an arbitrary fresh unprogrammed u2fzero and initialize it to look like the one you lost. It has to be done this way (at user's direct control) because if you buy u2fzero's pre-programmed all security is lost.

The secret blob can be stored on your [secure] phone and backed up in a safe on a piece of paper, or any other storage scheme which a particular user may require to meet their needs. Personally, I would store it in icloud.

or if you used publickey, closer to @ecesena's proposal, you don't need to keep a secret. (at the expense of having to prepare the backup devices ahead of time.)

In fact I really appreciate you explaining all these details, and to be honest I need to find a decent amount of time to fully wrap my head around it; hopefully on the weekend I'll be able to. I doubt it will change my reasoning about my personal use case, but learning something new about security is never bad. Thanks for that! Hopefully I'll get back to you after learning more.

Also wanted to make 100% sure: are you arguing just about the u2f-zero being insecurely implemented, or the whole concept of backup with the cloned and securely-stored token also being bad? If we imagine that e.g. yubico starts producing matched pairs of tokens (I doubt it would, but still let's imagine), primary and backup with the boosted counter, which are implemented securely enough etc - would it be still bad in your opinion?

> are you arguing just about the u2f-zero being insecurely implemented,

yes. the concept of a pre-cloned token is a good one. my main point of contention with you is that in your threat model, a lost phone is a security problem, yet a lost u2f is not. you need to revise your threat model or revise your solution.

I realized that I could explain my concerns better in the article. My biggest issue with the phone is not that it could get hacked, but that I could just lose it together with my u2f token (because, you know, I always carry both phone and u2f token), and thus get myself locked out of my accounts. So it's not about somebody attacking me specifically to get my 2fa data, but just about some bad luck happening which results in losing both u2f token and a phone which was a backup for u2f token.

Instead of Google Authenticator we could use Authy, which synchronizes its database with the server, but Authy account could be recovered with SMS, which is anything but secure. I actually updated the article just now with that point.

I really want having a backup which is rock-solid secure and reliable, you know, more reliable than any other 2nd factor I have. So having a token bricked into the wall or something like that would work.

> you need to revise your threat model or revise your solution

When I have a chance to reimplement the same backup concept having something more secure than u2f-zero, then yeah I surely will revise the solution.

http://www.break-ic.com/

in short, MCUs are not secure devices. the additional protection is great for surface level protection, and certainly sufficient for most threat models. but in this case, we are talking about an authentication token that is obligated to keep secrets. one cannot claim "secure implementation" without secure key storage.

even the atecc508a doesn't really meet the definition of secure. eg, there is no eal certification. no reputable vendor would use it in a commercial security device. it's great for its target market though.

unfortunately, there is no way for a hobbyist to acquire actually secure chips. something like the atecc508a will have to do.

I doubt there is 100% secure device, you know, it's all about tradeoffs. Given enough time and resources, nearly anything can be hacked.

By the way, if an attacker has gained physical access to the primary token (and also to the another factor, a password, since token on its own is not too helpful), they don't need to hack it in any way: they can just use it to log into the account, add some other token and revoke the existing one.

you might want to read up on EAL certification. Yes, there is no 100% secure device, but "secure" is about resistance to attack and an actual secure element is very resistant, as well as tamper evident.

u2f-zero could be hacked in 10s flat if you prep ahead of time.

> if an attacker has gained physical access to the primary token ... they don't need to hack it in any way

but they need long term access. with this device, short term access is enough to learn the key and then i can get access at a time of my choosing. i can also learn the counter value and you won't notice that i have gained access.

I'm not sure of any methods to bypass the read protection on normal MCUs in a 10s "drive by" attack. AFAIK, the special companies that provide flash readout (http://www.break-ic.com/), do so by decapping the chip and using involved imaging techniques. I suspect they get good at identifying various flash technologies, many of which are common to many chips. But don't think it's feasible for a drive by.

The I2C eavesdropping shouldn't be an issue because the ATECC508A does apply a mask.

If I lose my primary token, how can I not notice that? I look at it a few times per day, and I use it not rarely either, so I can't see how I can not notice if I lose it (even if it was replaced by the similarly-looking device).

RMASK and WMASK, which smelled not useful to you, are there exactly to prevent this from happening.

the key managed here

https://github.com/conorpp/u2f-zero/blob/master/firmware/src...

and here

https://github.com/conorpp/u2f-zero/blob/master/firmware/src...

just means that the "actual key" (unmasked) only lives in MCU memory for a short time -- the time from when the mask is applied and then until return to caller and memory is cleared, in the enrollment case I linked. In the authenticate case, it lives quite a bit longer because the stack space used for key storage isn't zero'd.

The atecc508 doesn't use or know how to use the mask. The actual key used for the encryption is passed in the clear over i2c.

(note that the key derivation you suggest is wrong because of the extra xor masking.)

http://ww1.microchip.com/downloads/en/DeviceDoc/20005927A.pd...